1. Field of the Invention
The present invention relates to a method for fabricating a cathode structure for a cathode ray tube, and more particularly, to a method for fabricating an indirectly-heated cathode structure.
2. Description of the Related Art
An indirectly-heated cathode structure, as shown in FIGS. 1 and 2, is constructed such that a heater 3, which is a heat source, and a cap 1a or 1b, on which an electron emitting substance 4 is coated or installed, are separated, unlike a directly-heated cathode structure, in which a cathode material is directly heated by a filament.
In detail, a cathode structure shown in FIG. 1 includes a cap 1a, a sleeve 2, and a heater 3. The cap 1a is coated with a cathode material 4, which is an electron emitting substance, and the cap 1a fits over the top end of the sleeve 2, in which the heater 3 is disposed. The heater 3 has a filament 3a having a double helix structure coated with a heat-resistant insulating material such as alumina.
The cathode structure shown in FIG. 2 is constructed such that the cap 1b is inserted into the top end of the sleeve 2.
In the aforementioned cathode structures, the operating temperature of the cathode is in a range between 700xc2x0 C. and 900xc2x0 C. in the case of an oxide cathode material, while the temperature of the cathode is in a higher range between 1400xc2x0 C. and 1500xc2x0 C. in the case of a metallic porous body or a metal alloy.
Thus, the metal cathode structure requires a high current because high temperature must be generated by the heater 3. However, the metal cathode structure operating at such a high temperature is liable to cause a leakage current between a heater and a sleeve due to a breakdown in an insulating material 3b which protects the filament 3a. In particular, when the heater 3 is positioned in the center of the sleeve 2 and is deviated to one side rather than being equally spaced apart from the inner surface of the sleeve 2 and the cap 1a or 1b, there is a high probability of a breakdown. Also, in the case where the heater 3 is deviated toward one side within the inner space of the sleeve 2, the thermal distribution is not uniform throughout the sleeve 2 and the cap 1a or 1b, thereby increasing a local difference in the electron emitting density of the electron emitting source.
Therefore, it is very essential that the heater is properly positioned at the center of a sleeve after inserting the heater into the sleeve. However, conventionally, there has been no particular step or measure taken for positioning the heater in the center of the sleeve.
To solve the above problems, it is an object of the present invention to provide a method for fabricating an indirectly-heated cathode structure which can prevent current leakage from being generated due to a breakdown in a heater by positioning the heater at a desired position on a sleeve.
It is another object of the present invention to provide a method for fabricating an indirectly-heated cathode structure which can reduce a local difference in the thermion emission density by uniformly heating a sleeve and a cap.
Accordingly, to achieve the first object, there is provided a method for fabricating an indirectly-heated cathode structure, including the steps of inserting a heater formed in a predetermined shape into a space of a mold, injecting a thermoplastic resin into the space of the mold to mold the heater by the thermoplastic resin, inserting the heater molded by the thermoplastic resin into a sleeve having a cap on its top end, connecting the sleeve and the heater by means of a support body to assemble a cathode structure, and heating the cathode structure to remove the thermoplastic resin used for molding the heater.
Preferably, the space of the mold is shaped so as to correspond to the inner space of the sleeve. In particular, a protrusion for a spacer, locally contacting the sleeve, for positioning the heater at a desired position of the inner space of the sleeve, is preferably provided on the outer surface of a thermoplastic resin for molding the heater.